A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g. comprising part of, one or several dies) on a substrate (e.g. a silicon wafer) that has a layer of radiation-sensitive material (resist). In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, and so-called scanners, in which each target portion is irradiated by scanning the pattern through the beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Electrostatic clamps may be used in lithographic apparatuses operating at certain wavelengths, e.g. EUV, since at these wavelengths, certain regions of the lithographic apparatus operates under vacuum conditions. An electrostatic clamp may be provided to electrostatically clamp (i.e. hold) an object, such as a mask or a substrate (wafer) to an object support, such as a mask table or a wafer table, respectively. Such a clamp may be described as an object holder. Alternatively and/or additionally, the clamp may form part of an object holder. The object holder might sometimes be described, alternatively and/or additionally, as an object support.
The voltage which is applied to an electrostatic clamp may be considerable. For example, the voltage may be of the order of kilovolts. An insulating barrier is conventionally located over an electrode of an electrostatic clamp, the insulating barrier acting to insulate an object such as a substrate from the voltage applied to the electrode. One or more burls may be provided on the surface of the insulating barrier which may be provided with a conductive coating that reduces the clamping force in the region of the burls. Further, the burls may be interconnected by conductor lines provided on the surface of the insulating barrier.
A problem exists in that the conductive coating over the burls and the interconnecting conductor lines are known to emit electrons that accumulate as a static charge on the surface of the insulating barrier. Because of the insulating barrier this static charge cannot dissipate and instead accumulates over time and is superimposed on the high voltage provided by the electrode(s). This results in local variations in the clamping force that are undesirable. While the accumulated static charge can be removed by, for example, cleaning the clamp with isopropyl alcohol, this requires removing the clamp from within a high vacuum apparatus and this is not a practicable solution.
It is desirable to provide, for example, an improved electrostatic clamp which obviates or mitigates one or more of the problems of the prior art, whether identified herein or elsewhere.